1978 20r. High Idle, Dieseling, timing?
02-04-2017, 03:34 PM
#1
Registered User
Thread Starter
1978 20r. High Idle, Dieseling, timing?
Hi all,
My 78 has been running pretty good but the only issue I've been having with it is dieseling. Seems like every time I shut it off it hits at least a time or two, and sometimes even will run for a few seconds.
I decided this weekend to see what I could see. I fired it up, let it warm up, and then used some ATF to decarb. I poured into the carb while high idle for about 10-15 sec then poured in enough to stall the engine. After it sat for about 30 min I took it for a drive and drove it hard. Didn't get the smoke show I was expecting but it did smoke pretty good.
When I got home and shut it off it did diesel but not bad. The PO had put platinum plugs in and I read those can cause dieseling so I proceeded to change those out for some regular ol NGKs. The plugs looked normal but maybe on the hot side. I then proceeded to tinker with the carb. First things first, wanted to check the anti-diesel solenoid, pulled the plug to the solenoid and engine almost died, but continued to lope along for a few seconds, and then died. My idle screw is "all the way out", and will not idle any lower no matter what I do. I would say its idling around 900-1000 rpm so not bad but not great either. I fiddled with the mixture screw without much change but did end up richening it by about a quarter turn. Checked for vac leaks and found none. Throughout the testing it did not diesel once but after taking it for a 5 minute ride it did diesel
Since no adjustment could be made on the carb I decided to check to see if the throttle linkage was sticking. I put another turn on the throttle return spring and verified it returns all the way. I can lower the RPM by maybe 50rpm by twisting hard on the throttle linkage. I then thought of everything else that could cause a high idle, the only thing I could come up with was an advanced timing. I figured this could also account for the hot looking plugs. I didn't have a timing light but timed it by ear and got the idle down to a better level.
That was all good until I went for a drive. It had 0 power, I couldn't even get up my hill in second. It seemed like once I got it going it had okay power but very little power off the line. Also was hard to start. I set the distributor back to about where it was and then decided to look into my vac advance and whether it was working. This is where things got interesting.
I didn't change anything about my advance when I desmogged (CA originally). I left it hooked up to the same port on the carb and left the check valve in. When I pulled the hose off at the distributor, there was no vacuum at all no matter what I did with the throttle. I pulled the other end off at the carb and couldn't get any vacuum there either. I then discovered the check valve was essentially the wrong way, it would allow allow air to flow toward the advance and not away from it. I should mention here that my dist is not the usual kind with the two lines. The advance diaphragm on mine points down toward the water pump and only has one hose.
So I figured nothing to lose at this point so I flipped the check valve around and hooked the hose to the manifold. Started right up but now idles even higher than before (judging by ear I'd say 1200 rpm or so). I decided to go for a quick drive and man what a difference. Has what feels like twice the power even compared to when I started (I don't need to mention it had more than when I had reset the timing
). When I got back from the drive it did diesel a small amount but not bad.
So now I guess I am looking for some direction. Where should the vac advance be hooked to? Guessing I don't need that check valve. Obviously now the truck runs good but man is the idle high. I guess I may need to go back through the carb adjustments now that the advance is working. But there is still the issue that I cannot reduce the idle with the idle screw. Is this normal? I've never seen one like that before, most engines I am used to you can dang near kill with the idle screw. Maybe something hanging up on my fast idle cam? (FI adjustment screw did nothing as well)
My 78 has been running pretty good but the only issue I've been having with it is dieseling. Seems like every time I shut it off it hits at least a time or two, and sometimes even will run for a few seconds.
I decided this weekend to see what I could see. I fired it up, let it warm up, and then used some ATF to decarb. I poured into the carb while high idle for about 10-15 sec then poured in enough to stall the engine. After it sat for about 30 min I took it for a drive and drove it hard. Didn't get the smoke show I was expecting but it did smoke pretty good.
When I got home and shut it off it did diesel but not bad. The PO had put platinum plugs in and I read those can cause dieseling so I proceeded to change those out for some regular ol NGKs. The plugs looked normal but maybe on the hot side. I then proceeded to tinker with the carb. First things first, wanted to check the anti-diesel solenoid, pulled the plug to the solenoid and engine almost died, but continued to lope along for a few seconds, and then died. My idle screw is "all the way out", and will not idle any lower no matter what I do. I would say its idling around 900-1000 rpm so not bad but not great either. I fiddled with the mixture screw without much change but did end up richening it by about a quarter turn. Checked for vac leaks and found none. Throughout the testing it did not diesel once but after taking it for a 5 minute ride it did diesel
Since no adjustment could be made on the carb I decided to check to see if the throttle linkage was sticking. I put another turn on the throttle return spring and verified it returns all the way. I can lower the RPM by maybe 50rpm by twisting hard on the throttle linkage. I then thought of everything else that could cause a high idle, the only thing I could come up with was an advanced timing. I figured this could also account for the hot looking plugs. I didn't have a timing light but timed it by ear and got the idle down to a better level.
That was all good until I went for a drive. It had 0 power, I couldn't even get up my hill in second. It seemed like once I got it going it had okay power but very little power off the line. Also was hard to start. I set the distributor back to about where it was and then decided to look into my vac advance and whether it was working. This is where things got interesting.
I didn't change anything about my advance when I desmogged (CA originally). I left it hooked up to the same port on the carb and left the check valve in. When I pulled the hose off at the distributor, there was no vacuum at all no matter what I did with the throttle. I pulled the other end off at the carb and couldn't get any vacuum there either. I then discovered the check valve was essentially the wrong way, it would allow allow air to flow toward the advance and not away from it. I should mention here that my dist is not the usual kind with the two lines. The advance diaphragm on mine points down toward the water pump and only has one hose.
So I figured nothing to lose at this point so I flipped the check valve around and hooked the hose to the manifold. Started right up but now idles even higher than before (judging by ear I'd say 1200 rpm or so). I decided to go for a quick drive and man what a difference. Has what feels like twice the power even compared to when I started (I don't need to mention it had more than when I had reset the timing
). When I got back from the drive it did diesel a small amount but not bad.So now I guess I am looking for some direction. Where should the vac advance be hooked to? Guessing I don't need that check valve. Obviously now the truck runs good but man is the idle high. I guess I may need to go back through the carb adjustments now that the advance is working. But there is still the issue that I cannot reduce the idle with the idle screw. Is this normal? I've never seen one like that before, most engines I am used to you can dang near kill with the idle screw. Maybe something hanging up on my fast idle cam? (FI adjustment screw did nothing as well)
02-04-2017, 06:17 PM
#2
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Location: N of Okechobee Florida
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I've had idling issues on an old rig and found it to be a oil pan gasket of all things. Also for dieseling on the same truck I had to install a fuel pressure regulator and set it to just over 3.5
02-04-2017, 08:02 PM
#3
Registered User
Thread Starter
I've searched high and low and all I can figure is it must be something carb related. I'm going to check out the high idle and secondary linkages tomorrow to make sure nothing is sticking or binding up. But, it may be time to tear the carb apart as well.
02-04-2017, 09:38 PM
#4
The electronic pump, outside the tank, is still going to have an internal check valve to keep the line primed. Which will as a side effect keep the upstream line pressurized and require the regulator to return fuel and keep the upstream pressure manageable by the float and other valves. If nothing else it will lessen the spring fatigue and expand the time between rebuilding the carb.
PS easy enough to check with a helper, shut off the engine and clamp the feed line to your carb as quick as possible.
PS easy enough to check with a helper, shut off the engine and clamp the feed line to your carb as quick as possible.
Last edited by Co_94_PU; 02-04-2017 at 09:40 PM.
02-05-2017, 11:46 AM
#6
Registered User
Thread Starter
Well, I am pleased to say I have solved the problem. Honestly I'm glad I tore into it because had I not I likely would have never discovered the inoperative vac advance.
First thing this morning I went and got some new vac hose and a new air filter (the latter just because it looked dirty). I ran a new hose for the vac advance, which was about the only hose I had not replaced on the truck. I then started looking around the carburetor for anything that may prevent the throttle from closing all the way. I ended up finding a screw on the front of the carb, buried under the float bowl and kind of pointing upward. I was curious if this could cause the idle to go up. After some googling I found the following diagram on Autozones website.
Top left picture, you'll notice the "throttle positioner diaphragm" and related "throttle positioner adjustment screw". I believe these are CA only parts, and my understanding is when you are decelerating the computer sends a signal to a vacuum valve which causes the diaphragm to open the throttle just a hair, in order to reduce emissions. What ended up happening on mine is that the diaphragm wasn't working and the screw was effectively holding the throttle open. This wasn't an issue before as with no vac advance it idled OK.
I was able to turn this screw and get the idle down. Once I had the idle where I wanted it I turned the other (normal) idle screw until the idle speed increased. I then backed out the TPD screw about another 1/2 turn so that I can adjust the idle with the normal idle speed adjustment. Just out of curiosity I pulled the anti-diesel solenoid and the engine immediately died, which is what is to be expected.
The engine now idles at around 750-800 rpm (by ear) and purrs like a kitten. The engine has not ran this good since I got the truck. I even thought it had died a couple times when stopping because it was so smooth and quiet. I went for a 10 min drive putting the truck through its paces and no pinging so I think my timing is good. When I got home and shut off not any sign of dieseling
My hope is this thread may help somebody out having the same issues as me. If any specific questions feel free to respond here or send me a PM. Thanks to everybody that helped!
First thing this morning I went and got some new vac hose and a new air filter (the latter just because it looked dirty). I ran a new hose for the vac advance, which was about the only hose I had not replaced on the truck. I then started looking around the carburetor for anything that may prevent the throttle from closing all the way. I ended up finding a screw on the front of the carb, buried under the float bowl and kind of pointing upward. I was curious if this could cause the idle to go up. After some googling I found the following diagram on Autozones website.
Top left picture, you'll notice the "throttle positioner diaphragm" and related "throttle positioner adjustment screw". I believe these are CA only parts, and my understanding is when you are decelerating the computer sends a signal to a vacuum valve which causes the diaphragm to open the throttle just a hair, in order to reduce emissions. What ended up happening on mine is that the diaphragm wasn't working and the screw was effectively holding the throttle open. This wasn't an issue before as with no vac advance it idled OK.
I was able to turn this screw and get the idle down. Once I had the idle where I wanted it I turned the other (normal) idle screw until the idle speed increased. I then backed out the TPD screw about another 1/2 turn so that I can adjust the idle with the normal idle speed adjustment. Just out of curiosity I pulled the anti-diesel solenoid and the engine immediately died, which is what is to be expected.
The engine now idles at around 750-800 rpm (by ear) and purrs like a kitten. The engine has not ran this good since I got the truck. I even thought it had died a couple times when stopping because it was so smooth and quiet. I went for a 10 min drive putting the truck through its paces and no pinging so I think my timing is good. When I got home and shut off not any sign of dieseling
My hope is this thread may help somebody out having the same issues as me. If any specific questions feel free to respond here or send me a PM. Thanks to everybody that helped!
